This application claims the priority of German Patent Application No. 197 46 251.0, filed Oct. 20, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a system for steam reformation of a hydrocarbon with a reformer as well as an operating method therefor. Systems of this kind and operating methods therefor are used for example in mobile applications in fuel-cell operated motor vehicles for hydrogen reformation of methanol. Methanol is transported in liquid form in order to provide the hydrogen required for the fuel cells, thus a large-volume water reservoir can be eliminated. A compact design of the system having a relatively low weight and low cost and permitting rapid reaction to the typical frequent load fluctuations in vehicles is desirable. A high degree of efficiency and the ability to heat the system rapidly are also desirable. Since the hydrogen reformation reaction takes place endothermally in a suitable reaction chamber of the reformer, which contains a suitable catalyst material, the reformer must be kept at a suitable elevated temperature during operation.
Systems of this kind are already known in which certain system components are integrated into a common module to produce a compact design. Reformation systems are disclosed in JP 62138306 A, JP 63021203 A, and JP 63040701 A, in which a reformer and an evaporator connected upstream thereof are integrated into a common reactor module. A burner is also associated with the reactor module, in which a fuel is burned with a flame to heat the evaporator directly. Provision can also be made to heat the reformer using the hot combustion offgases from the burner.
In a reformation system disclosed in U.S. Pat. No. 5,516,344, a reformer is integrated together with a CO shift converter connected downstream thereof into a common module. A burner is associated with this module that burns a supplied mixture with a flame. Then the reformer and the CO shift converter among other things are heated by the hot combustion offgases.
In a combined reformation and shift reactor disclosed in EP 0 600 621 A1, the heat generated by a CO shift stage is utilized in a steam generator that is in thermal contact with the CO shift stage.
JP 4-325401 (A) discloses a reformation reactor system with two reformers connected in series. The upstream reformer is in thermal contact with a CO conversion stage that is supplied with the starting product stream from the reformer located downstream, so that the steam reformation reaction that takes place in the upstream reformer is heated by the heat generated in the CO conversion stage.
In WO 96/32188, an exothermal chemical reaction in a first reaction chamber and an endothermal chemical reaction in a second reaction chamber that is in thermal contact with the first reaction chamber by means of a heat-conducting partition is disclosed. The two reaction chambers are connected in series in terms of flow. To perform methane or methanol reformation reactions, it is particularly proposed to react the methane or methanol in the upstream reaction chamber exothermally by an oxidation reaction, and in the downstream reaction chamber endothermally by means of a steam reformation reaction.
EP 0 361 648 A1 discloses a hydrogen-generating reactor of the tube bundle type for example, which includes a reformer unit with two reformer stages connected in series and a CO shift unit connected downstream. In the transitional area between the first and second reformation stages, the starting product gas from the first reformer stage is partially burned, especially the remaining methane still contained therein, with the addition of a gas containing oxygen, such as air. The CO content rises sharply, but is then reduced once again in the following CO shift unit, with the CO shift unit preferably being divided into two zones of different temperatures in series.
JP 07126001 A discloses a system that includes a reactor module of the plate stack type. This reactor module contains an evaporator, a reformer, and a CO oxidizer. These integrated three system components are connected in series in a direction transverse to the stack as a first group of second plate layers. A burner is provided adjacent to the evaporator in which an added mixture is burned with a flame. The hot combustion offgases are conducted in parallel with the reformation gas stream through a second group of second plate layers of the plate stack that forms a heat transfer structure. The second group of second plate layers alternate with those of the first group. As a result, the combustion offgases heat the evaporator, reformer, and CO oxidizer.
The object of the present invention is to provide a system for steam reformation of a hydrocarbon that is built in very compact form and at relatively low cost, and has a high degree of efficiency as well as high dynamics, together with a method for operating such a system.
The system according to an embodiment of the present invention includes an oxidizer/burner unit connected downstream from the reformer and in thermal contact therewith. The oxidizer/burner unit functions as both a CO oxidizer and simultaneously as a catalytic burner during reformation reaction operation of the reformer. In this manner, the functions of removal of carbon monoxide from the reformate gas and the heating of the reformer are achieved by this single combined oxidizer/burner unit. This allows the system to have an extremely compact design. As a result of the catalytic burner function, the reformer can be kept at the temperature required for reformation reaction operation by means of flameless combustion. As a result of the CO oxidation function, the CO concentration in the reformate gas can be reduced to a desired value, which is important for example during use of the reformate gas stream, which consists essentially of hydrogen, to supply the anode side of a fuel cell system in a motor vehicle operated by fuel cells, since excessively high CO concentrations can damage the catalyst material in the fuel cell system.
Because the system can be very compact, its space requirements are correspondingly low. The small volume of the reactor module composed of the reformer and combined oxidizer/burner unit and the resultant short gas flow pathways allow high dynamics for the system, thereby allowing rapid reaction to load fluctuations. Because of the low total weight of this reactor module and the fact that its elements can be heated, it can be warmed up rapidly during a cold start and therefore is able after only a short time to perform reformation under full load. Since the system can be made compact, its surface is also comparatively small, which in turn keeps heat losses low.
According to a method for operating this system, a gas containing oxygen with an oxygen component is added to the combined oxidizer/burner unit. The oxygen component is larger than the amount required solely for selective CO oxidation. This increased oxygen supply therefore has the result that not only is the carbon monoxide contained in the reformate gas sufficiently oxidized (the oxidation being already initiated when a certain amount of heat is generated) by the combined oxidizer/burner unit, but also a suitable fuel material is burned catalytically without a flame with the rest of the oxygen. In this connection, it is primarily the hydrogen contained in the reformate gas and any unreacted hydrocarbon present in the reformer that serves as the fuel. The addition of oxygen to the combined oxidizer/burner unit is controlled or regulated such that all of the heat that is produced by CO oxidation and catalytic combustion is just sufficient to keep the reformer at its operating temperature.
In a system according to another embodiment of the present invention, the reformer and the combined oxidizer/burner unit are integrated as a module having a heat transmitting structure into a reactor module of the plate stack and/or tube bundle type. In addition, an evaporator connected upstream of the reformer and a catalytic burner that is in thermal contact with the reformer are integrated into the reactor module as an additional module having a heat transfer structure. This produces a highly integrated reformation system with an evaporator, reformer, and CO oxidizer, as well as heating of the evaporator and reformer by catalytic combustion processes in combustion chambers that are in thermal contact with the latter. In another embodiment of this system, the two modules are separated from one another by thermally insulating elements and hence are decoupled thermally from one another.
In another embodiment of the present invention, at least one additional CO removal stage without a burner function is connected downstream from the combined oxidizer/burner unit, so that as a result of this additional CO removal stage, the CO concentration in the reformate gas can be reduced if necessary.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.